1. Field of the Invention
The invention is generally in the field of hydraulic cement used in the manufacture of concrete, more particularly to hydraulic cements that include pozzolans.
2. Relevant Technology
“Roman cement” was used by the Romans to build spectacular buildings and aqueducts that still stand after 2000 years. Roman cement was formed by mixing a pozzolan (e.g., volcanic ash or ground brick) with lime and water to form a lime-pozzolan cement. The hydration products of Roman cement are essentially the same as in modern Portland cement but they form much more slowly, making Roman cement impractical as a modern cementing material.
In modern concrete, pozzolans such as fly ash and volcanic ash are often used to replace a portion of Portland cement. Replacing a portion of Portland cement with pozzolan yields improved concrete with higher durability, lower chloride permeability, reduced creep, increased resistance to chemical attack, lower cost and reduced environmental impact. Pozzolans react with excess calcium hydroxide released during hydration of Portland cement and therefore help prevent carbonation. However, there is a limit to how much Portland cement can be replaced with pozzolan because pozzolans generally retard strength development.
Notwithstanding the potential economic and environmental benefits that would derive from increasing the pozzolan content and reducing the Portland cement content when manufacturing concrete, technical limits have limited their practical use to current levels. It is estimated that less than 40% of ready mix concrete in the United States uses any pozzolan at all and of those that do use pozzolans, the typical replacement level is about 10%-15%. While highly engineered concretes can include more pozzolan as a percentage of total binder, engineering concrete to overcome the deficiencies of blended cements comes at a high cost that is usually only justified in expensive building projects such as high rise buildings and large public works structures where the beneficial properties of the pozzolan outweigh the engineering costs. In most cases, the tendency of pozzolans to retard concrete strength development creates an upper replacement limit beyond which the advantages of Portland cement replacement disappear. In short, when cost and ease of manufacture are the chief concerns, such as in the case of general purpose concretes, pozzolans are typically used in low amounts or not all.